In the semiconductor industry or the like, ultrahigh purity gases are used. In recent years, as integration of circuits has progressed rapidly from ICs to LSIs to VLSIs, requirements in achieving ultrahigh purity of gases used in manufacturing processes for these semiconductors have become more stringent.
In addition, among various ultrahigh purity gases used in semiconductor manufacturing processes, highly-sensitive analyses of gas-phase moisture and xenon in oxygen, which are used in oxidation processes, and of trace amounts of gas-phase moisture in ammonia, which is used in formation of insulating nitride films, were difficult.
That is, as a method of analysis of trace components in a gas, a method in which an atmospheric-pressure-ionization mass spectrometer is used has been hitherto known. An atmospheric-pressure-ionization mass spectrometer is a mass spectrometer which is equipped with an ion source to perform ionization under atmospheric pressure. For example, when an analysis of a trace amount of moisture in nitrogen is conducted, since ionization of the nitrogen gas under atmospheric pressure allows charge transfer from the ionized main component ions (N.sub.4.sup.+) to coexisting water molecules (charge transfer reaction), and causes an increase in the number of ionized water molecules, highly-sensitive quantification of trace moisture becomes possible. The above reaction of charge transfer from the main component ions to the coexisting molecules occurs only when the ionization potential of the coexisting molecules is less than that of the main component, and therefore, a quantification of trace moisture in argon is possible based on a similar principle.
However, with regard to moisture in oxygen and to xenon in oxygen, since the ionization potential of oxygen (12.07 eV), which is the main component, is lower than the ionization potential of moisture (12.61 eV), which is a trace component, and is lower than the ionization potential of xenon (12.13 eV), such a charge transfer reaction as above does not occur. Accordingly, when an analysis of moisture in oxygen gas was performed in accordance with an analytical method in which a conventional atmospheric-pressure-ionization mass spectrometer was used, although a calibration curve of moisture having a mass number of 19 was obtained, the sensitivity was low; similarly, when an analysis of xenon in oxygen was performed, measurement in a range in which the concentration of xenon was low was difficult.
In addition, it has been known that oxygen and water form cluster ions (Anal. Chem. 51, 1447; H. Kambara, Y. Mitsui & I. Kanomata (1979)), and such cluster ions are uncontrollable in a conventional analytical method, which has also been a cause of difficulty in highly-sensitive analyses.
Furthermore, although in order to obtain a calibration curve of moisture, measurements have been hitherto conducted using a standard oxygen gas having a known moisture concentration which is filled in a container, there was a concern that oxygen might react with moisture in the container, and there was also a problem in that an accurate calibration curve could not be obtained since a standard gas in a container was not consistent at each use over a long period.
Similarly, with regard to moisture in ammonia, since the ionization potential of ammonia (10.16 eV), which is the main component, is lower than the ionization potential of moisture (12.61 eV), which is a trace component, such a charge transfer reaction as described above does not occur. Moreover, it has been known that ammonia also forms cluster ions with moisture, and a highly-sensitive analysis has been difficult (Japan Industrial Technology Association, Technical Data 169, 82, "Analysis of Trace Components According to API-MS"; Kenji KATO, Hiroshi TOMITA, and Noritaka SATO (1987)).